Ive been searching the gov website looking for advice on whether i need to file a tax return for 17-18.

I bought a property on a buy to let mortgage in july17 but didnt start renting it out until july18.
So as i have received no income i didnt think i would need to fill in a self assessment, but as the deadline approaches i'm starting to think maybe i should,
just to cover all my expenses?
I know i'll have to fill one in for 18-19, but most of my outgoings fall into the 17-18 dates.
Should i file this month?

In relation to what is presumably the commencement of your property letting business, the test is in my opinion when you first started to market the property as being available to let. But HMRC reckons it is when you actually first let the property - which was clearly 2018/19. Note that you can still claim all your pre-commencement expenses as having been incurred on the first day your business commenced so you will NOT lose out on any allowable expenses incurred beforehand.

You will need to register for Self Assessment by 05/10/19 for the 2018/19 tax year. That assumes your GROSS annual rental income before any expenses is more than £1,000. If not, you may choose to rely on the Property Allowance (but if you do and you made a net loss after expenses then you would forfeit the loss).